The present invention relates to a belt-type fixing device that is used in an electrophotographic image forming apparatus.
In Japanese Patent Laid-Open Publications 2001-356625, HEI 11-133776, and 2002-148979 has been disclosed a belt-type fixing device 100, as shown in FIG. 12, that has a rotatable pressurizing roller 101 having an elastic layer 101a composed of sponge or rubber on an outer circumference thereof, a fixing-belt supporting part 102, and an endless-sheet-like fixing belt 103 wound around the fixing-belt supporting part 102.
In the belt-type fixing device 100, contact part between the pressurizing roller 101 and the fixing belt 103 forms a fixing nip 104. The fixing belt 103 is brought into pressure contact with a nip forming member 102a of the fixing-belt supporting part 102 by the pressurizing roller 101, so that the fixing belt 103 is rotated in a direction of an arrow D by the pressurizing roller 101 that is driven to rotate in a direction of an arrow C. The pressurizing roller 101 is heated by a heater lamp 105 that is a heat source provided in the pressurizing roller 101, and a temperature of the pressurizing roller 101 is thereby raised to a specified fixation temperature (e.g., to 180° C.). After the temperature of the pressurizing roller 101 is raised to the specified fixation temperature, in the belt-type fixing device 100, a recording medium 107 on which an unfixed toner image 106 has been formed is introduced into the fixing nip 104 in a paper feeding direction shown by an arrow E, and the toner image 106 is heated and fixed on the recording medium 107 while the recording medium is passed through the fixing nip 104. The use of the nip forming member 102a that is fixed so as to be incapable of rotating is intended for forming the fixing nip 104 having a large width in order to ensure sufficient nip time, and the use of nip forming member 102a that has a low heat capacity is intended for reduction in warm-up time.
The belt-type fixing device 100, however, has a problem as follows. In the belt-type fixing device 100, a nip pressure distribution that is not uniform with respect to the paper feeding direction causes a difference in quantity of deformation in the elastic layer 101a in the fixing nip 104, thus leads to variation in conveying velocity for the recording medium 107 in the fixing nip 104, and thereby results in occurrence of image noise, increase in torque, and the like.
On condition that a thick paper such as cardboard is fed, the elastic layer 101a in the fixing nip 104 deforms more greatly on entrance side and exit side than in center part because a radius of curvature of the paper abutting on the pressurizing roller 101 becomes slightly smaller than a radius of curvature of an outer circumferential surface of the pressurizing roller 101. Accordingly, the nip pressure distribution is heightened at both ends with respect to the paper feeding direction and is lowered in the center part with respect to the paper feeding direction. As a result, a problem arises in that there are caused the variation in conveying velocity for the recording medium 107 and thus image noise.